The separation or removal of water from organic liquids is an important process within the chemical, petrochemical, and energy industries. Water removal is important in the primary production of a wide range of organic solvents, in the recovery and recycling of used solvents, and in the removal of water from chemical equilibrium reactions to drive the reaction towards a preferred product. Frequently, the removal of water is complicated by the formation of azeotropes of the solvent with water, precluding use of a simple distillation approach to produce an anhydrous solvent. The best example of such a problem occurs in the production of ethanol, where an azeotrope with ca. 95.6% ethanol and 4.4% water is formed. The formation of this azeotrope greatly hinders the production of anhydrous ethanol and significantly adds to cost of this solvent.
Various processes that have been used to dehydrate organic liquid streams include fractional distillation, fractional distillation using entrainers to overcome the azeotrope problem, adsorption processes, and newer membrane-based techniques such as pervaporation and vapor permeation.
Pervaporation is a process that involves a membrane in contact with a liquid on the feed or upstream side and a vapor on the permeate or downstream side. Usually, a vacuum or an inert gas is applied on the vapor side of the membrane to provide a driving force for the process. Typically, the downstream pressure is lower than the saturation pressure of the permeate. Vapor permeation is quite similar to pervaporation, except that a vapor is contacted on the feed side of the membrane instead of a liquid. As membranes suitable for pervaporation separations are typically also suitable for vapor permeation separations, use of the term “pervaporation” herein encompasses both “pervaporation” and “vapor permeation”.
The efficiency of a pervaporation membrane can be expressed as a function of its selectivity and of its specific flux. The selectivity is normally given as the ratio of the concentration of the better permeating component to the concentration of the poorer permeating component in the permeate, divided by the corresponding concentration ratio in the feed mixture to be separated:
  α  =                    y        w            /              y        i                            x        w            /              x        i            wherein yw and yi are the content of each component in the permeate, and xw and xi are the content of each component in the feed, respectively.
The trans-membrane flux is a function of the composition of the feed. It is usually given as permeate amount per membrane area and per unit time, i.e. Kg/m2 hr. In order to obtain a high trans-membrane flux, it is desirable to operate the pervaporation process at the highest possible temperature. However, this means that the membrane will be in contact with a feed mixture, which often has a high concentration of organic components, at high temperature. To achieve an economical lifetime for the membrane, it is preferable that all components of the membrane be durable under such demanding conditions.
In the pervaporation process, both mass and heat transfer occurs. The solution-diffusion model can describe mass transfer where the selectivity is determined by selective sorption and/or selective diffusion. For pervaporation dehydration membranes, selective sorption is governed by the presence of the active centers in the polymer that are capable of specific interactions with a polar fluid. Selective diffusion is governed by the rigidity and the regularity of the polymer structure and also by the construction of the polymer's interspace.
A variety of different types of membranes and membrane constructs have been described for use in pervaporation dehydration processes. The materials used to prepare the membranes include hydrophilic organic polymers such as polyvinylalcohol, polyimides, polyamides, and polyelectrolytes. In addition, inorganic materials such as molecular sieves and zeolites have been used.
Initially, polymer-based pervaporation membranes comprised dense, homogeneous membranes. Typical examples of such membranes are described by Yamasaki et al. [J. Appl. Polym. Sci. 60 (1996) 743-48]. These membranes suffer from low fluxes as they are fairly thick. While the flux of the membranes can be increased by decreasing the thickness of the membranes, this leads to a decrease in mechanical strength and robustness.
Two routes have been used to overcome the problem encountered by the above homogeneous membranes. The first route involves the use of an asymmetric membrane in which a dense surface layer is supported on a more porous material made from the same polymer. A typical example of such an asymmetric membrane is disclosed by Huang et al. [Sep. Sci. Tech. 28 (1993) 2035-48]. The second route involves the formation of a dense thin film on the surface of a suitable support membrane, wherein the chemical composition of the dense surface layer and the supporting membrane are typically different. Typically, the support membrane is an ultrafiltration membrane that may contain an incorporated fabric to provide additional strength. Examples of these thin film composite membranes are described in U.S. Pat. Nos. 4,755,299, 5,334,314, 4,802,988 and EP 0,381,477. In U.S. Pat. No. 4,755,299, a dense cross-linked polyvinyl alcohol layered composite membrane designed for dehydration of organic solvents is described as having a flux of 0.3 kg/m2 hr and a selectivity of 250 when separating a solution comprising 80% isopropyl alcohol (IPA) and 20% water at 45° C. One major disadvantage of these thin-film composite membranes, however, is their fragility. For example, the commonly used cross-linked poly(vinylalcohol) films supported on polyacrylonitrile ultrafiltration membrane supports are readily damaged through the formation of cracks in the films and through parts of the film falling away from the support. Great care must therefore be taken when mounting and using these membranes. It is also difficult to prepare such membranes in such a way that they are free of defects.
A special form of the thin-film composite membranes is referred to as a “Simplex” membrane. These are made up of thin films using alternating layers of oppositely charged polyelectrolytes. The membranes are made by successive immersions in solutions of the two different polylelectrolytes such that a multilayer complex is formed (see for example Krasemann et al. [J. Membr. Sci. 150 (1998) 23-30]; Krasemann et al. [J. Membr. Sci. 181 (2001) 221-8], and Haack et al. [J. Membr. Sci. 184 (2001) 233-43]). In Haack et al., a Simplex membrane with six double layers of poly(ethylenimine) and alginic acid has a selectivity higher than 10,000 and a flux of 0.3 kg/m2 hr in the pervaporation dehydration of 88 wt. % IPA at 50° C. While a high selectivity and reasonable fluxes can be achieved with the Simplex membranes, these membranes are complex to prepare as they require multiple coating steps. In order to get ideal performance, up to 60 dipping operations are sometimes needed. Another significant drawback lies in the fact that these membranes cannot tolerate feed water contents higher than 25% without loss of some of the multiple layers.
Mixtures or blends of oppositely charged polymers have been used to form homogeneous dense membranes. However, these membranes typically have low fluxes as they are relatively thick. Shieh and Huang [J. Membr. Sci. 127 (1997) 185-202] disclose homogeneous membranes prepared by casting a solution containing chitosan (positively charged) and polyacrylic acid (negatively charged), to form a polymer blend membrane. The thickness of the resulting membrane is between 20 μm and 40 μm and the performance of this membrane is poor in term of flux (flux of 0.03 kg/m2 hr and a selectivity of 2216 with a 95 wt. % ethanol/water feed at 30° C.). In another example, Lee et al. [J. Membr. Sci. 52 (1990) 157-72] use dense membranes comprising an interpenetrating polymer network (IPN) of two polymers of opposite charge (acrylic acid and polyurethane) for the pervaporation of ethanol/water mixtures. In this case, the swelling ratio of the cast film is controlled by cation/anion interactions between the two IPNs. This membrane has a rather low selectivity and moderate flux when used for the dehydration of ethanol/water solutions.
Zeolites and molecular sieves are known to have a high affinity for water. As a result, there has been considerable work focused on trying to incorporate zeolites as the active component or layer in a membrane, and thin zeolite films supported on ceramic membranes display very high fluxes and separation factors with water/alcohol mixtures. However, it is difficult to make these membranes free of defects because of cracking, and these membranes are expensive to prepare. Y. Morigami et al. [Sep. Pur. Tech. 25 (2001) 251-60] have described the first large-scale pervaporation plant using zeolite NaA membrane with tubular-type module. Berg et al. [J. Membr. Sci. 224 (2003) 29-37] prepared high performance zeolite A membranes with Titania support, with some membranes having 3.5 μm thickness and a selectivity of 54,000 and flux of 0.86 Kg/m2 hr when treating 95 wt. % ethanol/water at 45° C., which significantly outperforms other known membranes. This superior performance is ascribed to the pretreatment of the TiO2-support with UV-photons, which improves the hydrophilicity of the support and thus the attachment of the zeolite to the support. However, these membranes are very sensitive to the formation of defects caused by flaws in the support or by incorrect membrane handling. Compared with polymeric membranes, zeolite membranes are generally less swollen, more inert to chemicals and can endure high temperatures. However, zeolite membranes are brittle, and their cost is much higher than for polymeric membranes.
An alternative approach to the above involves the incorporation of zeolite particles into a polymeric support. Membranes of this type typically have low separation factors, particularly when the fluxes are maximized. For example, Zeolite NaA-filled poly(vinylchloride) (PVC) membranes were reported by Goldman et al. [J. Appl. Polym. Sci. 37 (1989) 1791-800]. By adding NaA zeolite absorbents, the performance of a PVC membrane was changed from a selectivity of 250 and a flux of 0.51 Kg/m2h to a selectivity of 7 and a flux of 5.68 Kg/m2h. Another example of absorbent-filled membranes was reported by Okumus et al. [J. Membr. Sci. 223(2003) 23-38] where zeolites (3A, 4A and 13X) were added as fillers to a base poly(acrylonitrile) (PAN) membrane. At optimum zeolite content, the flux is increased about nine-fold with a seven-fold loss of selectivity relative to homogeneous PAN membranes.
Another approach that has been used to form pervaporation membranes is a “pore-filled” construct. Mika et al. [U.S. Pat. No. 6,258,276] disclose that a cross-linked polyelectrolyte incorporated into the pores of a support member can be used for pervaporation dehydration. Membranes consisting of cross-linked poly(4-vinylpyridinium salts) exhibit reasonably high fluxes but very low separation factors in the dehydration of ethanol. These membranes consist of a single charged polymer. There are other reports of pore-filled membranes developed for pervaporation purposes. These include the work of Yamaguchi et al. [Macromolecules 24 (1991) 5522-27] in which methyl acrylate was grafted to the walls of an ultrafiltration (UF) membrane using a plasma activation process. These membranes, which consist of a single polymer within the pores of the membrane, have a flux of 0.5 Kg/m2 hr and a selectivity of 7 when treating 50 wt. % benzene/cyclohexane at 50° C. Ulbricht et al. [J. Membr. Sci. 136 (1997) 25-33] also describe a pore-filled membrane comprising a grafted acrylate. The impact of side-group functionality (hydrophilicity, size) and preparation parameters (monomer concentration, UV irradiation time) was analyzed using solutions comprising methanol and less polar hydrocarbons. Pervaporation tests with methanol/methyl tert-butyl ether (MTBE) at 50° C. were performed with these membranes, and for methanol feed concentration between 7 and 20%, the fluxes were between 0.75 and 1.5 Kg/m2 hr and the selectivity between 80 and 45. Hirotsu et al. [J. Appl. Polym. Sci. 36 (1988) 177-89] grafted acrylic acid and acrylamide comonomers into the pores of a polypropylene support member by plasma method, then treated these membranes with a sodium hydroxide solution to get the final ionized membranes. When dehydrating higher than 90 wt. % ethanol solutions, these membranes had both low fluxes and separations.
Membranes comprising pore-grafted copolymers have also been used to separate solutions. Frahn et al. [J Membr. Sci., 234 (2004) 55-65] describe the separation of aromatic/aliphatic hydrocarbons by photo-modified poly(acrylonitrile) supports, in which supports are grafted non-crosslinked linear copolymers that have, in some instances, both positive and negative groups. One of the problems faced in this system is that the grafted linear copolymers can undergo conformational changes, which changes can adversely affect performance. These membranes display a low separation factor and low fluxes when separating aromatic/aliphatic hydrocarbons. Another problem encountered with non-crosslinked systems is that amount of linear copolymer retained within the support member is quite low unless high light intensities or long irradiation times are used.
There is therefore a need for high performance membranes (high flux and high selectivity) that are robust, easy to fabricate, and low cost. Most of the known pervaporation membranes have either high flux or high selectivity, but not both features at the same time. Only zeolite based membranes have both higher fluxes and reasonably high selectivities, but they are more expensive than polymeric membranes, their fabrication is complex, and they are susceptible to cracking and to loss of performance.